SUMMARYThe widespread conviction that perceiving another person must rest on ambiguous and fakeable information is challenged. Arguing from biomechanical necessities inherent in maintaining balance and coping with reactive impulses, we show that the detailed kinematic pattern is specific to an acting person's anatomical makeup and to the working of his or her motor control system. In this way information is potentially available about gender, identity, expectations, intentions, and what the person is in fact doing. We invoke the lawfulness of human movement, as elucidated by recent advances in motor control theory, to demonstrate the virtual impossibility of performing truly deceptive movements and to argue in general terms for the specification power inherent in human kinematics.The outcome of the analysis is subsumed under a principle of kinematic specification of dynamics (KSD), which states that movements specify the causal factors of events. Generally, a linked multiple degrees-of-freedom system does not exhibit substitutability; a change in one of its "input" factors cannot substitute for, or cancel, the multivariable effects of a change in another factor.Six explorative experiments are reported. Displaying humans in action with Johansson's (1973) patch-light technique, we show that (a) the influence of an invisible thrown object on the kinematics of the thrower enable observers to perceive the length of throw; (b) the lead-in movements of a person lifting a box allow perception of what weight the lifter expects; (c) a person lifting a box cannot deceive observers about the weight of the box, only convey the deceptive intention; (d) gender of adults and children in complex activity is recognizable to about 75% of presentations; (e) gender recognition rises to about 85% correct when the observed persons are.not self-conscious about gender; (f) real gender and expressed (acted) gender are simultaneously, but independently, perceived; and (g) observer instructions to judge only "gender" yields results that erroneously indicate a deception effect.We conclude that the experiments have demonstrated the considerable effectiveness of kinematic information in enabling perception of persons and action. Judgments of good precision were often obtained. Some perceived properties were relatively subtle states of the seen person. True conditions were perceived despite deceptive endeavors. The KSD principle therefore appears an appropriate conceptual guide, and the patch-light technique a useful empirical method, for the study of social knowing.The concluding discussion argues that person perception has a dual nature in that true person properties and communicative or deceptive expressions are co-specified in the kinematic pattern. Hence, they constitute alternative foci for perception, and attention can switch freely between them in normal social interaction. Extensive parallels exist between this view of person perception and Gibson's (1979) treatment of pictures and picture perception.Furthermore, we argue that the stu...

The weight of a box can be seen by observing another person lifting and carrying it. Evidence is provided in two experiments, the first of which employed videotaped events with the actor and box visible only as 21 bright patches. Observers judged the weight of the box rather linearly with an average slope of .87 and with a pooled standard deviation of 3.8 kg. The second experiment compared visual and haptic perception of box weight in similar events under conditions of live action. Average slopes of 1.00 in the visual mode and 1.20 in the haptic mode were obtained with standard deviations of 3.1 kg and 2.0 kg, respectively. It is concluded that the weight of the box, as a dynamic variable of the event, is well specified in the kinematic pattern and hence in the optic array. Furthermore, the visual system is efficient in picking up such information.

Constructivist and Gibsonian approaches disagree over the possibility of direct perceptual use of advanced information. A trenchant instance concerns visual perception of underlying dynamic properties as specified by kinematic patterns of events. For the paradigmatic task of discrimination of relative mass in observed collisions, 2 mathematical models are developed, 1 model representing a direct, invariant-based approach, and 1 representing a cue-heuristic approach. The models enable a critical experimental design with distinct predictions concerning performance data and confidence ratings. Although pretraining results were mixed, the invariant-based model was empirically confirmed after a minimal amount of training: Competence entails the use of advanced kinematic information in a direct-perceptual ("sensory") mode of apprehension, in contrast to beginners' use of simpler cues in an inferential ("cognitive") mode.

A basic feature of some modem theories of perception is the notion of complex or higher order variables which are considered basic for perception. A distinction between "rote" and "smart" mechanisms is introduced and it is suggested that perception consists of smart mechanisms which directly register complex variables. A model of a perceiver, based on the smart polar planimeter, is constructed and used to illustrate the possible consequences of smart perceptual mechanisms for research in areas such as psychophysics, cognition, attention, and perceptual development and learning. 15, 17-34.

Novice observers differ from each other in the kinematic variables they use for the perception of kinetic properties, but they converge on more useful variables after practice with feedback. The colliding-balls paradigm was used to investigate how the convergence depends on the relations between the candidate variables and the to-be-perceived property, relative mass. Experiment 1 showed that observers do not change in the variables they use if the variables with which they start allow accurate performance. Experiment 2 showed that, at least for some observers, convergence can be facilitated by reducing the correlations between commonly used nonspecifying variables and relative mass but not by keeping those variables constant. Experiments 3a and 3b further demonstrated that observers learn not to rely on a particular nonspecifying variable if the correlation between that variable and relative mass is reduced.

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The visible kinematic pattern that occurs when objects collide contains information about the relative mass of the objects. Recently, Gilden and Proffitt (1989) have claimed that perceivers are limited to the use of simultaneous elemental cues and that therefore mass discrimination judgments are independent of whether the precollision epoch is visible or not. The present paper argues that their experimental results are irrelevant because crucial parameters were held constant, and that false conclusions were drawn because their data were not treated for what they are: threshold measurements obtained with the method of constant stimuli. Three experiments are reported, showing that a high level of performance is attainable only when the whole event is visible, and thus that the effective information is extended over time. In addition, modified PROBIT analysis was used to determine what information observers use when the precollision epoch is occluded. Analytically complex invariants, based on vector components in the abstract collision-axis system, proved to have the best fit to the data. Thus the elementaristic premises in Gilden and Proffitt's cue-heuristic model for relative mass perception are defeated.

The theory of direct perception holds that competent observers are able to detect optical patterns that specify the relative mass of colliding balls. Heuristic theorists, on the other hand, claim that judgments of relative mass are based on variables that do not specify relative mass. We contrasted these views with an experiment in which participants were given feedback on their ratio-scaled estimates of the relative mass of simulated colliding balls. Correlations between judged relative mass and various kinetic and kinematic measures of the collisions revealed that (1) judgments of relative mass become more accurate with feedback, (2) different observers use different variables, (3) during training, many observers change which variables they use, (4) before training, observers tend to use nonspecifying variables or combinations thereof, (5) after a minimal amount oftraining, at least some observers seem to detect mass-specifying information, and (6) the judgments do not support a generalization of the heuristic model of Proffitt (1989, 1994). These findings suggest that direct perception of relative mass is a skill that can be developed through appropriate training.Kinematics is the branch of mechanics that is concerned with aspects ofpure motion, such as position, velocity, and acceleration. Kinetics (or dynamics), in contrast, is the branch of mechanics that deals with motion by taking into account such variables as force, mass, and friction. Because kinetic properties of the environment are important for the guidance of action, it would be beneficial for animals to perceive them visually, and indeed, humans, for instance, have been shown to perceive kinetic properties (e.g., Michotte, 1963;Runeson & Frykholm, 1983). The fact that the optical array consists only of kinematic variables creates a problem: How are kinematic variables exploited in the perception of kinetic properties?The nature ofthis exploitation has been debated by proponents ofthe theory ofdirect perception (Runeson, 1995; Runeson, Juslin, & Olsson, in press; Runeson & Vedeler,The Netherlands Organization for Scientific Research (NWO) is gratefully acknowledged for funding this project. This research was conducted while D.MJ. was supported by a grant from the Foundation for Behavioural and Educational Sciences of this organization (575-12-070), awarded to C.F.M. S.R.'s participation and his laboratory resources were supported by the Swedish Council for Research in the Humanities and Social Sciences (HSFR). We also thank David Gilden, Geoffrey Bingham, and Harold Sedgwick for helpful reviews on earlier versions of this article. Correspondence concerning this article should be addressed to D. M. Jacobs, Faculty of Human Movement Science, Vrije Universiteit, van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands (e-mail: djacobs@fbw.vu.nl). , 1994Todd & Warren, 1982). The theory ofdirect perception assumes that kinetic properties of the environment are specified by kinematic patterns. Smart perceptual mechanisms are hypothesized for the...